Industrial fermentations predominately use glucose as a feedstock for the production of a multitude of proteins, enzymes, alcohols, and other chemical end products. Typically, glucose is the product of starch processing, which is conventionally a two-step, enzymatic process that catalyzes the breakdown of starch, involving liquefaction and saccharification. During liquefaction, insoluble granular starch is slurried in water, gelatinized with heat, and hydrolyzed by a thermostable alpha-amylase. During saccharification, the soluble dextrins produced in liquefaction are further hydrolyzed by glucoamylases.
Glucoamylase enzymes (glucan 1,4-α-glucohydrolases, EC 3.2.1.3) are starch hydrolyzing exo-acting carbohydrases, which catalyze the removal of successive glucose units from the non-reducing ends of starch or related oligo- and polysaccharide molecules. Glucoamylases can hydrolyze both the linear and branched glucosidic linkages of starch (e.g., amylose and amylopectin). Glucoamylases are commercially important enzymes and have been used in a wide variety of applications that require the hydrolysis of starch. For example, glucoamylases are typically used to produce fermentable sugars from the enzyme liquefied starch substrate. The fermentable sugars, e.g., low-molecular-weight sugars, such as glucose, may then be 1) converted to fructose by other enzymes (e.g., glucose isomerases); 2) crystallized; or 3) used in fermentations to produce numerous end products (e.g., alcohols, monosodium glutamate, succinic acid, vitamins, amino acids, 1,3-propanediol, and lactic acid). Fermentation and saccharification can be conducted simultaneously (i.e., an SSF process) to achieve greater economy and efficiency.
Glucoamylases have been used successfully in commercial applications for many years. Additionally, various mutations have been introduced in fungal glucoamylases, for example, Trichoderma reesei glucoamylase (TrGA), to enhance thermal stability and specific activity. See, e.g., WO 2008/045489; WO 2009/048487; WO 2009/048488; and U.S. Pat. No. 8,058,033. The need still exists for providing new glucoamylase variants with more desirable properties.